One cylinder internal combustion engines are used in a wide variety of applications, including lawn mowers and hand held power tools. In many of these applications, a piston reciprocable in a horizontally oriented cylinder drives a vertically oriented crankshaft. In the case of a four stroke engine having a vertical crankshaft, the intake and exhaust valves may be located to the side of the cylinder. In this side valve or flat face arrangement, the camshaft is usually located at the side of the crankshaft, with the push rods located in the same general horizontal plane in which the piston reciprocates.
Another conventional four cycle engine design utilizes overhead valves located in the cylinder head. Typically, the two push rods in this design are located one on top of the other at the side of the cylinder. Lubricating oil from the crankcase is pumped or otherwise directed to the valve train components housed in a rocker box at one end of the push rods, and returns to the crankcase through a return path located under the cylinder. A variation of this design is disclosed in U.S. Pat. No. 4,881,496 to Kronich, in which oil is directed through the upper push rod tube to the rocker box, and returns to the crankcase through the lower push rod tube.
Another prevalent feature of conventional small engines is that the crankshaft is usually supported in the crankcase on both sides of the cylinder axis. In one variation of this design, the crankshaft is integrally formed with two webs, between which a cracked connecting rod is assembled to the crankshaft with a bearing. This construction is expensive to make and assemble, and also presents difficulty in lubricating the connecting rod bearing by splash lubrication. In another variation of this design the crankshaft is formed from several discrete components. The lower end of the connecting rod is fit onto a crankpin, which is thereafter built up with the remainder of the crankshaft. While the connecting rod bearing is generally easier to lubricate in this construction, the engine as a whole is no less expensive to make or assemble.
It is recognized that overhead valve engines produce less harmful emissions than side valve engines. However, many overhead valve engines operate with approximately 20-30% higher mean effective pressure than side valve engines, and have approximately a 10-30% higher mean piston speed where the mean effective pressure is the highest. This means that an overhead valve engine can produce the same power with better thermal efficiency, lower piston displacement, and less weight than an equivalvent side valve engine.
Although naturally aspirated overhead valve engines are most efficient when operating at a mean piston speed of about 4-10 meters per second, the resulting crankshaft speed is impractical to use in many applications such as driving the cutting blade of a lawnmower. One solution to this problem is proposed by U.S. Pat. No. 4,583,504 to Morris, which shows a lawn mower gear reduction system including a crankshaft and crankgear which drive an output gear and an output shaft. The output gear drives a camshaft, while the output shaft drives a lawnmower blade at a lower speed than the speed at which the crankshaft revolves.
It is also recognized that internal combustion engines can be adapted to run on alternative fuels, such as liquified petroleum gasses, that produce less emissions than conventional fuels like gasoline or diesel fuel. In a typical alternative fuel engine, the liquified gas is sent from a gas bomb through a filter in the liquid state, and at least partially vaporizes upon entry through a lock valve into a vaporizer. Because the liquified fuel absorbs heat from the surroundings when it changes states, however, some provision must be made to prevent the effects of this vaporization, such as the formation of ice, from interfering with the operation of the engine.
This is normally accomplished by diverting a portion of the engine coolant to the parts of the engine that require warmth. U.S. Pat. No. 4,335,697 to McLean, for instance, discloses a system in which the liquified petroleum gaseous fuel tank and the line supplying the fuel to the carburetor are maintained at a constant temperature via a by-pass of the engine coolant circuit. This approach is not feasible in small engines, however, which are predominantly air cooled as opposed to water cooled.
A further problem frequently found in conventional internal combustion engine powered garden tools such as lawn mowers is that the air used to cool the engine is exhausted out the lower part of the engine. For example, U.S. Pat. No. 4,890,584 to Tamba et al. discloses an engine having a vertical crankshaft in which a cooling fan is fixed with a flywheel to an output shaft below the engine body. Cooling air is drawn over the engine, and is discharged from openings below the engine. The cooling air discharged in this manner may stir up soil or gravel lying on the ground around which the garden tool is operating, and may also interfere with the collection of clippings produced by a lawn mower or line trimmer.